1. Field of the Invention
The present invention relates to an optical fiber amplifier, a method of amplifying optical signals, and an optical communication system.
2. Related Background Art
Recently, erbium-doped optical fiber amplifiers for the 1,580-nm band (referred to as L-band EDFAs hereinafter) have been studied. This study is made to achieve WDM transmission in the 1,580-nm band (L band). For dispersion-shifted fibers, this WDM transmission is used to avoid nonlinear effects such as four-wave mixing. According to the WDM transmission, the capacity of WDM transmission can be increased using optical fiber amplifiers for the existing 1,550-nm (C-band) in parallel.
The present inventor has examined various studies while studying the L-band EDFAs, and found the following matter.
Reference 1 (Electron Lett. Vol. 34, No. 15, pp. 1,509-1,510, 1998) discloses as follows: L-band EDFAs require higher pumping power because it has a lower power conversion efficiency than that of C-band EDFAS.
To compensate for this power conversion efficiency, the following researches have been made. One research, as described in reference 2 (Optoelectronics and Communications Conference 98, 16C2-4, 1998), shows: (a) additional pumping light is supplied from an additional 1,550-nm band optical pumping light source to an erbium-doped optical fiber (EDF). This EDFA needs an additional 1,550-nm band optical pumping light source, so that the number of parts for the EDFA increases and extra power is required for the optical pumping light source. Another research, as described in reference 3 (Photon Technol. Lett. Vol. 11, No. 1, pp. 42-44, 1999), shows: (b) amplified spontaneous emission (to be referred to as ASE hereinafter) generated in 1,550-nm band is used for optical amplification in an erbium-doped optical fiber (EDF) which is not pumped by pumping light from a light source.
The latter includes two types of configurations. In one of the configurations, (b1) first and second EDFs are connected in series, and 1,480-nm pumping light is supplied to the junction of the EDFs so as to pump the only first EDF. Then, only forward ASE light, generated in the first EDF, is supplied to the second EDF in this configuration. In the other configuration, (b2) first and second EDFs are connected in series, and 1,480-nm pumping light is supplied to the junction of the EDFs so as to pump the only second EDF. Then, only backward ASE light, generated in the second EDF, is supplied to the first EDF in this configuration.
These EDFAs can improve the power conversion efficiency. Each EDFA, however, needs an additional EDF which is not pumped by pumping light. In addition, these EDFAs can use only one of the forward ASE light and backward ASE light.
It is an object of the present invention to provide an optical fiber amplifier, a method of amplifying an optical signal, and an optical communication system, which can use both forward ASE light and backward ASE light.
An optical fiber amplifier according to the present invention comprises an optical amplification portion and optical circulator portion. The optical fiber amplifier has an input terminal and output terminal. The optical amplification portion has an input port provided so as to receive pumping light, an output port, and an optical fiber device provided between the input and output ports. Erbium is doped to the optical fiber device. The optical circulator portion has a first port coupled to the input terminal, a second port coupled to the input of the optical amplification portion, a third port coupled to the output from the optical amplification portion, and a fourth port coupled to the output terminal. In the optical circulator portion, light from the first port is supplied to the second port, light from the second port is supplied to the third port, and light from the third port is supplied to the fourth port.
When pumping light and signal light enter the optical fiber device through the input of the optical amplification portion, forward ASE light and backward ASE light are generated. The forward ASE light is used for optical amplification in the optical fiber device. The backward ASE light is provided to the output of the optical amplification portion through the second and third ports of the optical circulator portion. Hence, the backward ASE light is also used for optical amplification in the optical fiber.
The optical fiber amplifier according to the present invention comprises an optical amplification portion and optical circuit device. The optical circuit device is provided between the optical amplification portion and the input and output terminals. The 1,580-nm band light can pass through the optical circuit device from the input terminal to the input port of the optical amplification portion. The 1,580-nm band light can pass through the optical circuit device from the output port of the optical amplification portion to the output terminal. The 1,550-nm band light can pass through the optical circuit device from the input port to the output port of the optical amplification portion, vice versa. The 1,580-nm band light cannot, however, pass through the optical circuit device between the input port and the output port of the optical amplification portion.
In this optical fiber amplifier, the 1,550-nm band backward ASE light from the input of the optical amplification portion is supplied through the optical circuit device to the output of the optical amplification portion. Hence, the optical amplification portion utilizes both the forward ASE light and backward ASE light.
According to another aspect of the present invention, there is provided a method of optically amplifying signal light using an optical fiber device doped with at least erbium. The method comprises the steps of: (1) providing the signal light and light to a first terminal portion of the optical fiber device; (2) obtaining ASE light from the first terminal portion of the optical fiber device; (3) providing the ASE light to a second terminal portion of the optical fiber device, and (4) obtaining amplified signal light from the second terminal portion of the optical fiber device.
When pumping light is supplied to the optical fiber device, forward ASE light and backward ASE light is generated therein. The backward ASE light is obtained from the first terminal of the optical fiber device and supplied to the second terminal of the optical fiber device. Thus, both the forward ASE light and backward ASE light are utilized for optical amplification in the optical fiber device.
An optical communication system according to the present invention comprises one or more optical fiber amplifiers, and a plurality of optical transmission lines connected to the input and output terminals of the optical fiber amplifiers. The optical fiber amplifiers have configuration as described below or already described above.